1. Field of the Invention
The present invention relates to a radiographic imaging device that captures a radiographic image expressing radiation that has passed through an imaging subject, a method of controlling detection sensitivity to radiation irradiation start and a storage medium stored with a program.
2. Description of the Related Art
Recently, radiation detectors such as Flat Panel Detectors (FPDs) are being implemented in which a radiation sensitive layer is disposed on a Thin Film Transistor (TFT) active matrix substrate and with which radiation can be converted directly into digital data. Radiographic imaging devices such as electronic cassettes that employ such radiation detectors to capture radiographic images expressing irradiated radiation are also being implemented. Conversion methods for converting radiation into electric signals used by such radiation detectors include for example indirect conversion methods, in which radiation is first converted into light with a scintillator and then the converted light is converted into charge by a photodiode, or direct conversion methods in which radiation is converted into charge with a semiconductor layer containing for example amorphous selenium. There are various materials that may be used in the semiconductor layer for each method.
In radiographic imaging devices equipped with FPDs, it is necessary to perform synchronization control between the FPD and a radiation source in order to match the start of an accumulation operation, in which the FPD accumulates signal charge, to an irradiation timing of irradiation of radiation from the radiation source. In order to synchronize the timing for the start of radiation irradiation and the timing for the start of the accumulation operation of signal charge by the FPD, a controller such as a console that controls the radiographic imaging device receives an irradiation start signal generated by an irradiation switch connected to the radiation source and supplies this signal to the radiographic imaging device as a synchronization signal. The radiographic imaging device transitions to the accumulation operation and starts imaging on receipt of this synchronization signal.
However, in cases where an imaging system is configured including a radiographic imaging device and a radiation source, sometimes a synchronization control interface installed as standard in the radiographic imaging device or the console thereof (for example cable or connector standards, synchronization signal format) is not compatible with an interface of the radiation source. Due to such issues, radiographic imaging devices are being developed that include an automatic radiation detection function, with radiation irradiation start automatically detected by the radiographic imaging device itself, without the use of a synchronization signal.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2011-185622 discloses a radiographic imaging device provided with: plural radiation detection elements arrayed in a 2D formation in each region of regions partitioned by plural scan lines and plural signal lines; current detection means that detects current flowing in a bias line for applying a bias voltage to the radiation detection elements; control means that detects radiation irradiation start based on a value of the current detected by the current detection means; and memory pre-stored with change profiles of the current detected by the current detection means during reset processing of each of the radiation detection elements. The control means detects radiation irradiation start based on a value ΔV that is the value of the current detected by the current detection means during the reset processing of each of the radiation detection elements reduced by a value corresponding to a value of the current in the change profile.
Moreover, JP-A No. 2011-193306 discloses technology in which, at a point in time when radiation is definitely not being irradiated in a radiographic imaging device, acquiring image data d (offset correction value O) and an integrated value Σd(n) or a summed value Σd(m) for each image data d from each of radiation detection elements 7, or acquiring an integrated value Σd(n) or summed value Σd(m) for plural frames' worth of image data d and computing average values thereof. A threshold value for detecting radiation irradiation start each time radiographic imaging is performed by is then set by increasing these values by adding a specific value.
In a radiographic imaging device (referred to below as an electronic cassette) with an automatic radiation detection function such as disclosed in JP-A No. 2011-185622, there is an issue of false detection of radiation irradiation start due to noise that has been incorporated into the radiation detection system. Conceivable noise sources are, for example, dark charge occurring inside the electronic cassette, magnetic fields and electromagnetic waves emitted from external devices such as Magnetic Resonance Imaging (MRI) devices, and noise caused externally for example by vibration of a table on which the electronic cassette is installed. Of such noises, the noise generation state from noise sources present inside the electronic cassette, such as dark charge, is not expected to fluctuate greatly. Namely, the amplitude fluctuation of noise occurring inside the electronic cassette is expected to be comparatively small, leading to comparatively little variation in noise level. It is accordingly possible to avoid the above false detection by providing a fixed margin to a threshold value for determination of radiation irradiation start.
However, it is foreseen that the level of noise incorporated into the radiation detection system of the electronic cassette from noise sources external to the electronic cassette, for example electromagnetic waves emitted from external devices and vibration, will fluctuate greatly depending on such factors as where the electronic cassette is installed and the time of day. Namely, the amplitude fluctuation of external noise caused by external factors is expected to be comparatively large, with comparatively large variation in the noise level. Accordingly, when the electronic cassette is installed in a noisy environment that is affected by external noise sources, it is foreseen that even if a fixed margin is provided to a threshold value for determination of radiation irradiation start, noise will still occur at a level that exceeds the margin amount. Such cases lead to the false detection of radiation irradiation start. There is therefore the concern of frequent false detection of radiation irradiation start due to noise when the electronic cassette is installed in a noisy environment affected by external noise sources, with the results that the transition to an accumulation operation cannot be made at an appropriate timing, and that radiographic imaging cannot be appropriately performed.
Moreover, even when as disclosed in JP-A No. 2011-193306, at a point in time when radiation is definitely not being irradiated in a radiographic imaging device, image data d and an integrated value Σd(n) or a summed value Σd(m) for each image data d is acquired, average values thereof are computed, and a threshold value for the detection of radiation irradiation start is set by then adding a specific value to these computed values, it is foreseeable that there will be large fluctuations in the noise level after the threshold value has been set, in which case there is still a concern of false detection of radiation irradiation start.